Public access cellular systems are currently available for use in major markets in the U.S., Canada, and numerous foreign countries. The ability to communicate with a land line telephone system from a portable or mobile cellular unit has become a valuable business asset.
As a result of experiences with public cellular systems, corporate interest has emerged in being able to combine radiotelephony with private land line corporate telephone systems. Such systems, so called "private cellular systems" need to be able to provide service in a coverage area to only authorized users. In many instances, the coverage region of private cellular system must simultaneously coexist with any overlapping region of a public system and not interfere therewith.
This problem is exacerbated by the fact that it is more cost effective to use the same subscriber or user equipment in both systems than to have special transceivers for the private system. Using the same subscriber equipment for both the public and the private system introduces potential frequency allocation conflicts.
When it is necessary to service movable units in a private system that is in the coverage area of a public cellular carrier, there are two conflicting problems. If the RF coverage area of the private system is limited or spotty, it is possible for the subscriber units or mobiles that are supposed to lock onto the private system to instead lock onto service in the surrounding public system.
If, however, the private system saturates the area with RF (to ensure capture of the subscriber units), its coverage area will expand, and as a result, the private system will capture subscriber units that should be locked to the public system.
This results from the operation of subscriber or movable cellular units which will always scan for and lock onto the strongest control channel. If a subscriber unit is locked onto the wrong system, it will not be possible for it to make or receive calls on the desired system.
Previously, special programming of the subscriber units and the private system was required to attempt to control access to the private or limited access system. In these prior attempts, either non-standard control frequencies, or non-standard system identification (SID) codes were used. The prior approach has been and continues to be unattractive since it requires the use of features that may not be present in all manufacturers' cellular subscribe units. In some systems, the regulatory requirements for allowing such special operation may be excessive to the point of impracticality.
Thus, there continues to be a need for private cellular-type systems which can provide limited access in the coverage area of a public cellular system. Preferably, the private system subscriber units would be the same as those of the public system.
In addition, it would be desirable to be able to provide public system service to a public system subscriber unit which happens to be within the coverage area of the private system. Conversely, it would also be desirable for a private system subscriber unit to be able to use the public cellular system.